U.S. patent application number 12/370428 was filed with the patent office on 2009-09-24 for method for selecting subchannel mode and mimo communication system using the same.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Jinyin XUE, Jie ZHANG, Hua Zhou.
Application Number | 20090238297 12/370428 |
Document ID | / |
Family ID | 40848472 |
Filed Date | 2009-09-24 |
United States Patent
Application |
20090238297 |
Kind Code |
A1 |
ZHANG; Jie ; et al. |
September 24, 2009 |
Method For Selecting Subchannel Mode And MIMO Communication System
Using The Same
Abstract
The invention relates to Method for Selecting Subchannel Mode
and MIMO Communication System Using the Same. A method for
self-adaptively selecting a code modulation sub-channel mode is
suitably used in a MIMO communication system that comprises a base
station and mobile terminals, wherein the method comprises the
steps of: the mobile terminal estimating channel state information
thereof, and determining a sub-channel structure most suitable for
data transmission thereto, based on the channel state information;
the mobile terminal feeding back information on the determined
sub-channel structure most suitable for data transmission thereto
to the base station; and the base station determining a sub-channel
structure used for a down link, based on the information on the
sub-channel structure fed back by the mobile terminal. According to
the present invention, the subchannel structure in the
communication system can be self-adaptively changed according to
the actual communication environment, so as to improve the entire
communication quality and efficiency.
Inventors: |
ZHANG; Jie; (Beijing,
CN) ; Zhou; Hua; (Beijing, CN) ; XUE;
Jinyin; (Beijing, CN) |
Correspondence
Address: |
HANIFY & KING PROFESSIONAL CORPORATION
1055 Thomas Jefferson Street, NW, Suite 400
WASHINGTON
DC
20007
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
40848472 |
Appl. No.: |
12/370428 |
Filed: |
February 12, 2009 |
Current U.S.
Class: |
375/267 |
Current CPC
Class: |
H04L 1/0001
20130101 |
Class at
Publication: |
375/267 |
International
Class: |
H04L 1/02 20060101
H04L001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2008 |
CN |
200810082926.4 |
Claims
1. A method for self-adaptively selecting a code modulation
sub-channel mode in a MIMO communication system that comprises a
base station and mobile terminals, wherein the method comprises the
steps of: the mobile terminal estimating channel state information
thereof, and determining a sub-channel structure most suitable for
data transmission thereto, based on the channel state information;
the mobile terminal feeding back information on the determined
sub-channel structure most suitable for data transmission thereto
to the base station; and the base station determining a sub-channel
structure used for a down link, based on the information on the
sub-channel structure fed back by the mobile terminal.
2. The method according to claim 1, wherein the mobile terminal
determines the sub-channel structure most suitable for data
transmission thereto, based on any one of the following a) to d):
a) to make the fading on the determined sub-channel as flat as
possible; b) to make the effective SINR in one sub-channel as high
as possible; c) to make the average physical SNR on the determined
sub-channel as high as possible; d) to balance among a), b) and
c).
3. The method according to claim 1, wherein the mobile terminal
feeds back index of the determined sub-channel structure,
pre-coding vectors selected based on the sub-channel structure, and
calculated value of channel quality indicator, to the base
station.
4. The method according to claim 1, wherein the base station
determines a sub-channel structure used for a down link, according
to service requirement, service quality, power limitation, delay
requirement or in a way of maximizing the throughput, based on the
sub-channel structure fed back by the mobile terminal.
5. The method according to claim 1, wherein if there are a
plurality of the mobile terminals, the base station determines the
sub-channel structure used for the down link, according to
priorities of the mobile terminals or in a way of maximizing the
total throughput, based on the sub-channel structures fed back by
the mobile terminals.
6. The method according to claim 5, wherein if one mobile terminal
is mandatory transmitting terminal, the base station selects a
sub-channel structure fed back by said one mobile terminal as the
channel structure used for the down link.
7. A MIMO communication system wherein the method according to any
one of claims 1 to 6 is applied.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to self-adaptively selection
of a code modulation subchannel mode in a MIMO communication
system.
BACKGROUND OF THE ART
[0002] In multiple-user multiple-input-multiple-output (MU-MIMO)
downlink transmission, a base station (node B) is provided with M
transmitting antennas, and K (K.gtoreq.M) users are provided with
N.sub.k receiving antennas, when channel state information at
transmit side (CSIT) is available, closed loop transmission shall
be adopted to increase the spectral efficiency via multiple
antennas. But in narrow bandwidth applications of the feedback
link, the base station cannot accurately acquire the CSIT, thus a
limited feedback strategy based on quantization shall be applied in
such beamforming based close-loop systems. In MIMO-OFDM systems, as
the number of subcarriers in one OFDM symbol is large (usually more
than 512), and the channel fading on different subcarriers are not
equal to each other exactly, the amount of feedback for beamforming
over all data subcarriers are enormous, which makes the close-loop
transmission impossible.
[0003] One possible scheme to the above is to divide the
subcarriers in time-frequency grid into several blocks, in which
the same beamforming vector is applied to all the subcarriers
belonging to the same block, i.e., subchannel. The possibility of
this method depends on the following two factors. Firstly, the
channel fading fluctuation in time-frequency grid is directly
influenced by the time and frequency selective fading
characteristics, and fortunately, in most of the practical
applications, the fluctuation over one subchannel is very slight,
therefore the channel fading over one subchannel can be regarded as
approximately flat. Secondly, the common codebook at both the
mobile terminal MS and the base station B is discrete as it just
consists of 2.sup.N.sup.b precoding vectors, wherein N.sup.b is bit
number for characterizing the whole codebook. Even though the
channel fading on the subcarriers belonging to the same subchannel
has slight fluctuation, the selected precoders may be still
suitable for all the subcarriers in this subchannel.
[0004] The two points above make the block-wise precoding scheme
practical in real systems with slowly time-varying fading, and each
subchannel in time-frequency grid shall consist of the subcarriers
with similar fading as much as possible, thus the structure of the
block shall be deliberately designed. Presently, the structure of
subchannel of the downlink for transmitting data from the base
station B to the mobile terminal MS is fixed.
SUMMARY OF THE INVENTION
[0005] The present invention is provided in view of the above
problem in the prior art, and the aim of the present invention is
to self-adaptively select a subchannel structure adapted to the
downlink in the MIMO communication system, according to the actual
communication environment.
[0006] One aspect of the present invention provides a method for
self-adaptively selecting a code modulation subchannel mode in a
MIMO communication system that comprises a base station and mobile
terminals, wherein the method comprises the steps of: the mobile
terminal estimating channel state information thereof, and
determining a subchannel structure most suitable for data
transmission thereto, based on the channel state information; the
mobile terminal feeding back information on the determined
subchannel structure most suitable for data transmission thereto to
the base station; and the base station determining a subchannel
structure used for a down link, based on the information on the
subchannel structure fed back by the mobile terminal.
[0007] Preferably, in the above method, the mobile terminal
determines the subchannel structure most suitable for data
transmission thereto, based on any one of the following a) to d):
a) to make the fading on the determined subchannel as flat as
possible; b) to make the effective signal-to-interference-noise
ratio (ESINR) in one subchannel as high as possible; c) to make the
average physical signal plus interference-to-noise ratio (SINR) on
the determined subchannel as high as possible; and d) to balance
among a), b) and c).
[0008] Preferably, in the above method, the mobile terminal feeds
back index of the determined subchannel structure, precoding
vectors selected based on the subchannel structure, and calculated
value of channel quality indicator, to the base station.
[0009] Preferably, in the above method, the base station determines
a subchannel structure used for a down link, according to service
requirement, service quality, power limitation, delay requirement
or in a way of maximizing the throughput, based on the subchannel
structure fed back by the mobile terminal.
[0010] In the above method, if there are a plurality of the mobile
terminals, the base station determines the subchannel structure
used for the down link, according to priorities of the mobile
terminals or in a way of maximizing the total throughput, based on
the subchannel structures fed back by the mobile terminals.
[0011] In the above method, if one mobile terminal is mandatory
transmitting terminal, the base station selects a subchannel
structure fed back by said one mobile terminal as the channel
structure used for the down link.
[0012] Another aspect of the present invention further provides a
MIMO communication system wherein the method for self-adaptively
selecting a code modulation subchannel mode is applied.
[0013] According to the present invention, the subchannel structure
in the communication system can be self-adaptively changed
according to the actual communication environment, so as to improve
the entire communication quality and efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 schematically shows a MIMO communication system of
the present invention.
[0015] FIGS. 2A and 2B show two examples of the subchannel
structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] In the present invention, three criteria for block design
are adopted so that the total throughput of the base station B is
approximately maximized. The first criterion is to minimize channel
fluctuation, the second one is to maximize effective SINR, and the
third one is to maximize precoding output power of a subchannel.
The three criteria are all performed at the mobile terminal MS
side. Moreover, the subchannel structure is selected at the base
station B to meet the priority requirement between different users,
or maximize the throughput.
[0017] Multiple user transmission in MIMO downlink can boost the
system throughput as well as improve quality-of-service (QoS). The
CSIT acquirement is not an easy task for downlink beamforming in
closed loop transmission. Codebook based feedback is an effective
approach to overcome the inaccuracy in the CSIT while possessing
favorable performances. If the precoding vector index feedback is
based upon subcarrier, the amount of feedback overhead is
tremendous, which makes an uplink channel feedback impossible. The
block-wise precoding and feedback strategy is commonly adopted in
real systems. Although the channel fading in one subchannel is not
exactly the same, the structures of subchannels under different
channel environments shall be easy to be investigated.
[0018] Different users suffer from different channel fading, and
this makes the subchannel structures between them inconsistent with
each other. The selection of the subchannel structure at the base
station B shall also be designed to support the MU-MIMO
transmission in high spectrum efficiency systems.
System Model
[0019] In downlink MU-MIMO systems, the base station B deploys M
transmitting antennas, there are total K users waiting for
communication with the base station B simultaneously at the same
frequency band. Herein K>1 corresponds to space-division
multiple access (SDMA) transmission, and K=1 corresponds to SU-MIMO
system. The diagram of such communication system is shown in FIG.
1.
[0020] In FIG. 1, each terminal estimates its own channel state
information (CSI) respectively. According to the estimated CSI, the
terminals determine their respective subchannel structures most
suitable for data transmission thereto. The criteria used here can
be such that the channel fading is fat on the predetermined
subchannel as much as possible, or that the effective SINR of the
subchannel is maximized. Based on the predetermined subchannel
structure, each user selects the best precoding vector in the
N.sup.b-bit codebook according to maximization of the receiving
SNR, and calculates the channel quality indicator (CQI) value.
After this procedure, the terminals feed back their respective
subchannel structure indices, selected precoder indices and CQI
values to the base station B. The base station B jointly determines
the only subchannel structure for all selected J users' downlink
transmissions. The criteria used herein can be maximizing sum
capacity, proportional fairness and QoS guarantee etc. The
parameter J shall satisfy
j = 1 J K j .ltoreq. M ##EQU00001##
so as to avoid multi-user interference as much as possible.
Furthermore, the base station B selects suitable modulation coding
style (MCS) for each terminal according to QoS requirement. In some
advanced applications, power allocation between different users is
used to further improve the spectral efficiency. The precoding
module applies different precoding weight to transmit data streams
for different users.
[0021] The focus of the invention is to determine the subchannel
structure, i.e., select a most suitable structure from a set
containing several possibilities.
Feedback at the Mobile Terminal Side
[0022] Assuming the codebook set known to both the base station B
and each terminal is denoted by
S = [ c 0 , c 1 , c 2 N b ] , ##EQU00002##
and the channel state information from the base station to user k
is denoted by H.sub.k.epsilon.C.sup.M.times.K.sup.k whose element
is unit covariance complex Gaussian distribution in zero mean. In
applications, we assume that each user can estimate its channel
state information H.sub.k accurately. For convenience, we assume
that the noise powers at all terminals are the same, say
.sigma..sub.n.sup.2. Then the user k selects the best codebook
vector according to the following maximizing SNR criteria.
w k = arg max c l .di-elect cons. S ( H k H c l 2 2 ) ( 1 )
##EQU00003##
[0023] The by-product CQI value is obtained from
CQI.sub.k=.parallel.H.sub.k.sup.Hw.sub.k.parallel..sub.2.sup.2
(2)
[0024] As the codebook set S is common to the base station B and
all the mobile terminals, the K mobile terminals feed back
respective precoding codebook index (PVI) of selected vector from S
to the base station B via the dedicated feedback uplink
channel.
Subchannel Structure Selection
[0025] For OFDMA systems, the resource in time-frequency grid is
divided into many rectangular subchannels, and the average channel
coefficient over one subchannel between the m.sub.t.sup.th
transmitting antenna of the base station B and the m.sub.r.sup.th
receiving antenna of the user k is computed as follows:
H _ m t , m r = t = 1 S t f = 1 S f H m t , m r t , f ( 3 )
##EQU00004##
wherein H.sub.m.sub.t.sup.t,f,.sub.m.sub.r is the channel fading of
the f.sup.th subcarrier and t.sup.th OFDMA symbol between the
m.sub.t.sup.th transmitting antenna of the base station B and the
m.sub.r.sup.th receiving antenna of the user k; S.sub.tS.sub.f=S is
the total number of subcarriers in one subchannel. Here the uplink
feedback is based upon subchannel such that the total amount of
feedback is dramatically decreased, wherein SN.sup.b bits are
required for feedback per subcarrier whereas N.sup.b bits for
feedback per subchannel. One subchannel occupies several
subcarriers in frequency domain and several OFDMA symbols in time
domain, and the number of subcarriers and symbols in one subchannel
with fixed number of total subcarriers shall be well designed to
maximize the system performances.
[0026] Here the subchannel structure shall be selected based on the
channel characteristics. FIGS. 2A and 2B depict two examples of
subchannel structure. FIG. 2A spans more OFDMA symbols in time
domain, whereas less subcarriers in frequency domain. As the
subchannel in time-frequency grid is rectangular, the total number
of subcarriers in a subchannel can be decomposed by the product of
several pairs of integers. Assuming S has L possible decompositions
as follows,
S=S.sub.t.sup.1S.sub.f.sup.1=S.sub.t.sup.2S.sub.f.sup.2= . . .
=S.sub.t.sup.LS.sub.f.sup.L (4)
Method 1
[0027] The first criterion of selecting subchannel structure is to
make the channel fading over a subchannel as flat as possible. The
following minimum correlated covariance for all possible
decompositions in Eq. (4) is adopted in this invention, i.e.,
( S t opt S f opt ) = arg min ( S t l , S t f ) m rt = 1 M T m r M
R t = 1 S t l f = 1 S f l ( | H m t , m r t , f | 2 - 1 S t = 1 S t
l f = 1 S f l | H m t , m r t , f | 2 ) 2 m rt = 1 M T m r M R t =
1 S t l f = 1 S f l | H m t , m r t , f | 4 ( 5 ) ##EQU00005##
Method 2
[0028] The second criterion is to maximize the effective SINR
(ESINR) over a subchannel. For all channel fading values of a
specific subchannel structure, the SINR of the subchannel is
calculated by adopting the mapping method from link level to system
level. In the present invention, the following common mapping
method from link level to system level is adopted:
ESINR.sup.l=F(H.sub.1,1.sup.1,1,H.sub.1,1.sup.1,2, . . . ,
H.sub.1,1.sup.S.sup.t.sup.l.sup.,S.sup.f.sup.l) (6)
wherein F is the adopted mapping method from link level to system
level; in practical system design, methods such as EESM, MMIB or
RBIR can be used. Eq. (6) describes the calculation method of ESINR
with respect to SISO condition, and in case of MIMO condition, the
calculation of ESINR depends on the adopted MIMO transmission
technology. Then the subchannel structure maximizing ESINR is
obtained via Eq. (6).
( S t opt S f opt ) = arg max ( S t l , S t f ) ESINR l ( 7 )
##EQU00006##
Method 3
[0029] The third criterion is to maximize the average physical SNR
over a subchannel. For all the alternatives of subchannel
structures, we shall select the corresponding precoders based on
the average channel coefficient, and find out the subchannel
structure having the maximum average SNR. For a specific subchannel
structure (S.sub.t.sup.l,S.sub.f.sup.l), the precoder is determined
based upon the following average channel,
H _ m t , m r l = t = 1 S t l f = 1 S f l H m t , m r t , f ( 8 )
##EQU00007##
[0030] Then, the precoding vector index is obtained from
w l k = arg max c l .di-elect cons. C ( H _ l H c l 2 2 ) ( 9 )
##EQU00008##
[0031] The real average SNR of the current subchannel based on
(S.sub.t.sup.l, S.sub.f.sup.l) is
SNR.sub.k.sup.l=.parallel.
H.sub.l.sup.Hw.sub.l.sup.k.parallel..sub.2.sup.2 (10)
[0032] Therefore, the subchannel structure is determined by finding
maximum of the following optimization problem,
( S t opt S f opt ) = arg max ( S t l , S f f ) SNR _ k l ( 11 )
##EQU00009##
[0033] After each user determining the best subchannel mode, the
corresponding PVI for precoding and CQI, all these information is
fed back to the base station B.
Processing at the Base Station B
[0034] Assuming the feedback channel is error-free and zero
feedback delay. The base station B demodulates the information on
precoding vector indices and CQIs and individual subchannel
structure from all users, then determines the subchannel mode to be
used actually, active user set, i.e., the set contains the user
indices which are allowed for downlink data transmission.
[0035] For all the subchannel modes fed back from individual user,
the base station B selects the real subchannel structure according
to user priority and the maximum number of the same subchannel
mode, i.e., if a user with subchannel mode A is mandatory user for
transmission, the base station B selects the subchannel mod A as
the subchannel structure. Otherwise, the base station B selects the
one required by most users as the subchannel structure. In case of
a single user, the subchannel structure is determined according to
the feedback information of the user of interest, and the
beamforming vector is the precoder indexed by PVI.
[0036] In case of multiple users, selection of effective users and
beamforming can be based on greedy algorithm.
[0037] Although the present invention is only illustrated with the
preferred embodiments, persons skilled in the art can easily make
various changes and modifications without going beyond the scope of
the invention defined by the claims, on the basis of the contents
disclosed herein. The description of the above embodiments is just
exemplary, and does not intend to limit the invention defined by
the claims and the equivalents.
[0038] All contents of the following references are incorporated
into the Description of the application by reference, as described
in details herein. [0039] [1] Part 16: Air Interface for Fixed
Broadband Wireless Access Systems, IEEE P802.16 (Draft March 2007),
Revision of IEEE Std 802.16-2004, as amended by IEEE Std
802.16f-2005 and IEEE 802.16e-2005 [0040] [2] 3GPP R1-072422, NTT
DoCoMo, "Investigation on precoding scheme for MU-MIMO in E-UTRA
downlink" [0041] [3] 3GPP R1-060335, Samsung, "Downlink MIMO for
EUTRA" [0042] [4] 3GPP R1-060495, Huawei, "Precoded MIMO concept
with system simulation results in macrocells" [0043] [5] 3GPP
R1-062483, Philips, "Comparison between MU-MIMO codebook-based
channel reporting techniques for LTE downlink" [0044] [6] 3GPP
R1-060362, NES Group, "MIMO techniques for downlink E-UTRA:
detailed description and simulation results" [0045] [7] IEEE
802.16m-07/037r2, "Draft IEEE 802.16m Evaluation Methodology"
[0046] [8] 3GPP R1-063028, Phillips, "System-level simulation
results for channel vector quantization feedback for MU-MIMO"
* * * * *